1. Field of the Invention
The present invention relates to a mold powder for continuous casting which enables manufacture of a slab having excellent surface quality in the casting of steel at a high casting speed. The present invention also relates to a continuous casting method using the mold powder of this invention.
2. Description of the Related Art
Steel plates for use in outer parts of automobiles, which require excellent surface quality and high cold workability, are generally manufactured from continuously cast slabs having a thickness of about 250 mm. This is because slabs having both excellent surface quality and internal quality are required, along with high slab productivity. The casting speed is generally 2-3 m/min. A casting speed greater than this results in defects, including longitudinal cracks on the surface of a slab and residual non-metal inclusions in the slab.
On the other hand, for the past several years, there has been developed a mini-mill in which a thin slab continuous casting machine and a simple hot rolling machine are disposed in a same production line. In the method of thin slab continuous casting, from the viewpoint of productivity, a casting speed of at least about 5 m/min is the target speed to attain.
Since molten medium carbon steel causes peritectic reaction when solidified, the surface of a slab made of such steel is susceptible to longitudinal cracks. If the medium carbon steel is cast by thin slab continuous casting at a high casting speed, longitudinal cracking on its slab surface is accelerated. Also, low alloy steel often suffers longitudinal cracks on its slab surface if the low alloy steel contains an alloy element which increases susceptibility to cracking.
It is well known that a strong relationship exists between longitudinal cracking on the slab surface and mold powder. In continuous casting, molten steel is fed into a mold through an immersion nozzle, and a mold powder is applied onto the surface of the molten steel in the mold under casting. Typically, the mold powder comprises a mixture of one or more types of oxides and carbon powder or others. The mold powder added to a mold is melted by the heat of the molten steel, to thereby form a molten slag layer on the surface of the molten steel. The molten slag layer flows into the gap between a mold surface and a solidified shell, to thereby form a slag film. The slag film is cooled while in contact with the mold surface and solidified. The solidified slag film is composed of crystal and glass.
The mold powder showing the above properties has the following effects:
1) maintaining temperature of the molten steel surface in a mold and preventing oxidation of the molten steel, PA1 2) absorbing bubbles and inclusions which are present in molten steel and float on the surface thereof, PA1 3) securing lubrication between a mold surface and a solidified shell, and PA1 4) adjusting cooling rate of a solidified shell through adjusting heat resistance of molten slag layer. PA1 (A) If CaO'/SiO.sub.2 is not less than 0.9 and not greater than 1.9, the CaF.sub.2 content is 15-60 wt. %, or PA1 (B) If CaO'/SiO.sub.2 is greater than 1.9 and not greater than 2.8, the CaF.sub.2 content is 5-60 wt. %, PA1 T.CaO: the total Ca content in the mold powder as converted to CaO (wt. %), and
Of these effects, item 3), securing lubrication, is particularly important in casting at high casting speed. Also, item 4), adjusting cooling rate of the solidified shell within a mold, i.e. a slab surface, in the early stage of solidifing, is important for preventing longitudinal cracks on a slab surface.
Generally, if casting speed is increased in continuous casting, the amount of molten slag flowing into the gap between a mold surface and a solidified shell decreases. If the amount of molten slag flowing into the gap and the slag film thickness decrease accordingly, the solidified shell is restrained on the mold surface due to poor lubrication. In the worst case, this may lead to operational accidents such as breakout. In order to secure the amount of molten slag flowing into the gap, the solidification temperature of the molten slag is lowered or the viscosity of the molten slag is lowered. However, lowered solidification temperature or viscosity of molten slag often impairs uniformity of the thickness of a slag film. Consequently, the cooling rate of the solidified shell within the mold lacks uniformity, so that the slab becomes susceptible to longitudinal cracks. Therefore, an important technical objective is to create a mold powder which imparts both lubrication, which is effective in increasing the casting speed, and uniformity of cooling rate of a solidified shell, which is effective in prevention of longitudinal cracks on a slab surface.
For prevention of longitudinal cracks on a slab surface in casting at high casting speed, the following methods have been proposed: In Japanese Patent Application Laid-Open (kokai) No. 3-193248 is proposed a method of incorporating into mold powder, as accelerators of crystallization, oxides of elements belonging to Group IIIA and Group IV such as ZrO.sub.2, TiO.sub.2, Sc.sub.2 O.sub.3, and Y.sub.2 O.sub.3. Further, according to this invention a high casting speed is achieved if the viscosity of a molten slag is lowered to 1 poise or less at 1300.degree. C. In the course of being cooled, the molten slag produces crystals which mildly cool the slab surface within a mold. This mild cooling of the slab surface makes the cooling rate of the slab surface uniform.
Also, in Japanese Patent Application Laid-Open (kokai) No. 5-15955 are proposed lowering the viscosity of a molten slag and increasing the ratio by weight of T.CaO to SiO.sub.2 (T.CaO/SiO.sub.2). In this method, T.CaO represents a sum of the amount of CaO contained in the mold powder and the amount of CaO converted from Ca-containing matter assumed to be present in the mold powder in the form of CaF.sub.2, and is defined by the following equation (Z). EQU T.CaO (wt. %)=CaO (wt. %)+CaF.sub.2 (wt. %).times.(56/78) (Z)
In this invention, if T.CaO/SiO.sub.2 is increased to as much as about 1.2 to 1.3, crystallization occurs in the cooling process of a molten slag, by which the slab surface within the mold is cooled mildly.
However, in the case where the mold powders proposed in the above-mentioned Japanese Patent Application Laid-Open (kokai) Nos. 3-193248 and 5-15955 are used in casting medium carbon steel containing peritectic steel, if the casting speed exceeds a certain value, mild cooling effect of a molten slag to the mold surface becomes insufficient, resulting in longitudinal cracking on the slab surface. Briefly, use of these mold powders at a casting speed of about 2-3 m/min or more may result in longitudinal cracking on the slab surface.
Apart from the above-mentioned inventions, a mold powder having a high T.CaO/SiO.sub.2 ratio is proposed in Japanese Patent Application Laid-Open (kokai) No. 5-269560. This mold powder has a CaO/SiO.sub.2 ratio of 1.1-1.8, and has a CaO/F ratio of 9-40.
Further, in Japanese Patent Application (filed in 1978) Laid-Open (kokai) No. 54-35129, a mold powder having a relatively high CaO/SiO.sub.2 ratio is proposed. This mold powder has a CaO/SiO.sub.2 ratio of 0.6-1.4, and contains fluorspar in an maximum amount of 10 wt. %.
As mentioned above, in order to prevent longitudinal cracks on a slab surface in casting medium carbon steel, low alloy steel, or low carbon steel at a casting speed of about 5 m/min or higher, the performance of mold powder is required to be further improved.
An object of the present invention is to provide a mold powder which is advantageously used in high-speed casting such as thin slab continuous casting, or in continuous casting of steel such as medium carbon steel which is susceptible to longitudinal cracks.